Conventionally, gallium nitride (GaN)-based semiconductor materials represented by a compositional formula, such as AlxGayInzN (0≦X, Y, Z≦1; X+Y+Z=1), and having a direct-transition-type bandgap of energy corresponding to a wavelength region of short-wavelength visible light to the UV region have been employed for fabricating pn-junction light-emitting devices, such as blue, green or UV LEDs and LDs (see, for example, JP-B SHO 55-3834).
Conventionally, the p-conduction-type GaN semiconductor layer for fabricating a pn-junction gallium nitride-based semiconductor light-emitting device is formed through addition of an element belonging to Group II of the periodic table serving as a p-type impurity (i.e., Group II impurity). For example, there has been disclosed a technique in which a Group II impurity, such as magnesium (Mg) or zinc (Zn), is added to a GaN layer through ion injection means (see, for example, JP-A SHO 54-71590).
However, without any further treatment, the gallium nitride-based semiconductor layer to which a Group II impurity has been added generally does not serve as a p-type conductive layer exhibiting high conductivity. One conceivable reason for this is that hydrogen (H) migrating from a growth atmosphere to the layer during vapor phase growth electrically compensates the Group II impurity, thereby deactivating the impurity. Thus, according to a conventional procedure, a gallium nitride-based semiconductor layer is formed through addition of a Group II impurity to the layer, followed by heating the layer in order to remove, as much as possible, hydrogen contained in the layer (see, for example, JP-A HEI 6-237012). Another known technical approach is irradiation with charged particles for electrically activating a Group II impurity (see, for example, JP-A SHO 53-20882).
Even when virtually the entire amount of hydrogen is removed from the GaN semiconductor layer to which a p-type Group II impurity has been added, the thus obtained low-resistance p-type conductor layer does not necessarily attain excellent, reliable rectifying characteristics and electrostatic blocking voltage characteristics when a pn-junction LED is fabricated therefrom. Among these characteristics, currently, consistent electrostatic blocking voltage is difficult to attain, even when the p-type GaN-based semiconductor layer is formed on a conductive substrate, such as a silicon (Si) single-crystal substrate, silicon carbide (SiC) substrate or gallium arsenide (GaAs) substrate.
An object of the present invention is to improve electrostatic blocking voltage of a gallium nitride-based semiconductor device, such as an LED, fabricated from the aforementioned conventional p-type GaN-based semiconductor layer by minimizing variation of blocking voltage and suppressing increase in resistance to as small an increase as possible.